1. Field of the Invention
The present invention relates to a technique for improving the characteristics such as the wear resistances of the surfaces of metallic parts such as automotive parts, e.g., camshafts or crankshafts or a variety of other mechanical parts and, more particularly, to a process for forming a dispersion alloy layer on a metallic base made of steel, cast iron, aluminum alloy or the like.
2. Description of the Prior Art
A dispersion alloy is prepared by dispersing particles of carbides, oxides, nitrides or hard metals in a metallic matrix. The dispersion alloy thus prepared is hard and has an excellent wear resistance so that it has been widely used as a reinforced alloy in a member required to have the wear resistance.
The most common one for preparing those reinforced dispersion alloys is the precipiration process for heat-treating a supersaturated solid solution, in which an element composing a dispersion phase is dissolved over the solid solution limit of a matrix metal at a room temperature, to precipitate the dispersion phase. Another known is the sintering process for compressing and sintering the mixture of powder forming the matrix and powder forming the dispersion phase. Also known is the internal oxidization, nitrization or carbonization process which uses a material containing a metal more reactive with oxygen, nitrogen or carbon, respectively, than the matrix metal. This material is held at a high temperature in oxidizing, nitriding or carbonizing gases so that these gases may be allowed to diffuse into the material to form the oxide, nitride or carbide in the material.
In another aspect, there are also known processes for forming a dispersion alloy layer locally in the surface of a metallic base. As is disclosed in Japanese Patent Laid-Open No. 32373/1982, for example, the surface layer of a metallic base is melted with a TIG arc or the like, and non-metallic powder such as oxide powder is blown and mixed into the molten pool to form in the base surface layer a layer in which the non-metallic particles are dispersed in the metal. Another but similar process using a laser beam in place of the TIG arc is disclosed in U.S. Pat. No. 4,299,860. In this connection, we have already proposed in Japanese patent application No. 137488/1985 a process for forming a dispersion alloy layer locally in the surface of a metallic base, by arranging the surface of the metallic base with powder of a metal having a higher affinity with oxygen than that base metal, by melting and mixing the powder and the surface layer of the metallic base with a highly concentrated energy such as a laser beam in the atmosphere of oxidizing gases to oxidize the metal of the powder thereby to form a dispersion alloy layer in which the oxide phase is dispersed.
Generally speaking, the excellent wear resistance required of metallic members used in the various mechanical parts is limited to merely a local portion of the metallic members, and the wear resistance per se is one of the properties belonging to the surface. It follows that it is frequently sufficient to give an excellent wear resistance exclusively to the surface layer. On the other hand, the dispersion alloy has a higher production cost than the ordinary alloys and is so hard that it is inferior in workability. As a result, a metallic member made of the dispersion alloy in its entirety would frequently be disadvantageous in the workability and production cost. It is, therefore, frequently desired that the member be made of the reinforced dispersion alloy not in its entirety but locally at its surface layer where the wear resistance is demanded. This tendency is prominent especially in the case of large-sized parts.
From the standpoint of the characteristics of the dispersion alloy, on the other hand, the dispersion-phase particles usually have the higher hardness for the finer size. From the standpoint of the wear resistance especially for the sliding friction, on the contrary, excessively fine dispersion-phase particles (of 10 .mu.m or smaller, for example) would be unable to ensure a sufficient wear resistance. It is, therefore, desirable that particles of a considerable size (of about 10 to 100 .mu.m) be dispersed.
Thus, examinations of the dispersion alloy making processes of the prior art have been conducted mainly from those standpoints to reveal that none of the processes could satisfy the above-specified demands sufficiently and could be freed from various drawbacks, as will be described in the following.
First of all, the precipitation process for precipitating the dispersion-phase particles by the heat treatment of a long time finds difficult the local reinforcement by the dispersion. Since the growth of the precipitate is promoted by the diffusion in the solid, the size of the precipitate is several microns at most even after the long heat treatment, and the fact is that sufficient sliding-wear resisting characteristics are not always obtained. Another problem is that the treatment itself takes a long time.
Next, the sintering process is allowed to set the size of the dispersion-phase particles freely because the mixed powder is compressed and sintered. Despite of this advantage, however, the sintering process is premised on the assumption that the material is made of a dispersion alloy as a whole for the compressing and sintering steps. This makes it difficult to form the dispersion alloy layer locally. Another problem is that the sintering step also takes a long time.
On the other hand, the internal oxidizing, nitriding or carbonizing process also requires a long time for the treatment because the diffusion of the gases in the solid is the determining rate. In addition, the necessity for the integral heating makes it difficult to form the dispersion alloy layer locally.
All the processes disclosed in Japanese Patent Laid-Open No. 32373/1982, U.S. Pat. No. 4,299,860 and Japanese patent application No. 137488/1985 are directed to the technique for forming the dispersion alloy layer locally. These processes are satisfactory in the point of local formation but are accompanied by the following problems, respectively.
The process of Japanese Patent Laid-Open No. 32373/1982 or U.S. Pat. No. 4,299,860 includes the step of merely blowing the non-metallic powder into the molten pool. As a result, the particles of the non-metallic powder are not dispersed uniformly in the metallic matrix but are frequently localized or agglomerated to raise a problem that a sufficient wear resistance is hardly attained. The dispersion-phase matrix metal is necessarily restricted to become identical to the parent material (or the base). This leads to another problem that the characteristics of the dispersion alloy layer never fail to be limited by the characteristics of the metal making the base.
On the other hand, the process of Japanese patent application No. 137488/1985 is accompanied by the reaction between the molten metal and the gases so that the size of the dispersion-phase particles is several microns at most to raise a problem that the sufficient sliding-wear resisting characteristics cannot be attained. Like the aforementioned process of Japanese Patent Laid-Open No. 32373/1982, the outstanding process is restricted by the fact that the matrix metal of the dispersion alloy layer necessarily has to be identical to the base portion.